Metageologisthttp://all-geo.org/metageologist
Geology: beautiful things, incredible ideasSun, 22 Mar 2015 22:03:28 +0000en-UShourly1http://wordpress.org/?v=4.1.1Into the Third Dimension: using Google Maps to know what’s undergroundhttp://all-geo.org/metageologist/2015/03/google-earth-into-the-third-dimension/
http://all-geo.org/metageologist/2015/03/google-earth-into-the-third-dimension/#commentsSun, 22 Mar 2015 21:53:29 +0000http://all-geo.org/metageologist/?p=3972Continue reading →]]>Much of the earth’s surface is covered by sedimentary rocks. These form as sediment settles on the surface. As the types of sediment change – sand to mud to sand again – different layers are formed, some hard some soft. The patterns these layers make are responsible for some of the most interesting Great Geology in Google Earth.

Sedimentary layers start off flat1 but as plates collide and squash, they may be folded, like pushing the edge of a rug. The resulting 3-dimensional structures are later eroded and brought to the surface – itself a 3-D structure. The 2-D lines we see on the aerial images below are formed by the intersections of these different 3-D structures. This can make interpreting them a little difficult, as we’ll see.

Compare and contrast the next two images. First look at these smooth lines from Mexico.

Versus these zig-zag ones from Argentina.

Both of these patterns involve sedimentary rocks, but the causes of the wavy lines are very different. The secret is to work out the shape of land surface and then infer the shape of the sedimentary layers. Rivers are our friends here. In Argentina there is a clear relationship between them and the lines the layers are making on the ground. Every zig has a river in it and every zag is a little hill in between. The sedimentary layers are pretty much flat and the pattern of lines is caused by the shape of the land surface. Imagine cutting a wedge out of a layered cake – this is what it would look like.

In Mexico the pattern is mostly due to folding of the sediments. The beautiful curves and swoops are due to flat layers having been slowly buckled as the earth’s plates rearranged themselves.

Analysing these shapes and making sense of them is bread-and-butter for geologists. A bed-rock part of any geological education. Typically we use maps, but in desert areas photos tell everything we need to know.

One important trick geologists learn is to create cross-sections, drawing a slice through the earth to show how the folded rocks continue underground. One of the many types of sedimentary layer is a coal-seam, so you can see how this is not a purely academic exercise.

A good geological map will have symbols showing how many degrees tilt the layers have and in which direction they are pointing down. Without these we can’t do a proper cross-section. But using our friends the rivers and streams we can still tell a lot.

Here’s a nice fold. Imagine one of the layers is rich in valuable unobtanium and you want to mine it2. You can trace a line where the edge of it is, but which side of the line is the rest on? Think of it another way, are we looking down at an arch with the top sliced of (an antiform) or a basin (a synform)3 Should you sink your unobtanium mine-shaft in the middle of the fold or around the edge? If you get it wrong, you’ll choose the part where the valuable layer has already been eroded away from.

Look at the rivers (streams/creeks) that cross the layers on the top side of the fold. Notice a pattern? Every time the stream cross the layers, it makes a little V-shape, with the sharp bit of the V pointing North. A stream bed is always lower than its surroundings so it gives us a glimpse into the third dimension. Cut into the layer and it’s edge moves north – it’s deeper the further north you – it’s dipping to the north – it’s a tilted sheet that disappears under the ground to the north.

We can check if we’re right because this a fold. For this trick to work the south side of the fold should have the opposite pattern. We are looking at a breached arch and the southern side should have the opposite pattern. It’s harder to see on this side (probably because the beds are tilted more nearly vertically and the effect is smaller) but indeed, our little V-shapes point the other way.

This 3-D stuff is hard work. Yet it’s something geologists have to be good at (maybe there’s some link with the strong anecdotal evidence that they tend to be left-handed). If you need some help, some of these Google Maps have good photos associated to help you get another view of the structures. Alternatively this post makes the same link.

Let’s leave you with some sheer aesthetic pleasure. Some totally flat layers turned into beautiful patterns by erosion.

]]>http://all-geo.org/metageologist/2015/03/google-earth-into-the-third-dimension/feed/2Looking from the sky at diamondshttp://all-geo.org/metageologist/2015/02/looking-from-the-sky-at-diamonds/
http://all-geo.org/metageologist/2015/02/looking-from-the-sky-at-diamonds/#commentsSun, 22 Feb 2015 12:33:10 +0000http://all-geo.org/metageologist/?p=3934Continue reading →]]>When a geologist looks at Google Maps images, we usually filter out any human activity. But in the case of mines, that would be a mistake – the holes humans dig can tell us about the geology.

What’s this? A big circular hole in the ground and large piles of bluish mine waste. The shadows are on the northern side, showing we are in the Southern hemisphere. This is the Letlhakane mine in Botswana.

And here’s another one, in the northern hemisphere. The hole here is half a kilometre deep (it’s an older mine, the Mirny mine in Siberia. They used jet engines to melt the soil so they could dig).

And again, the Diavik mine in Canada. Note that the hole is considerably deeper than the lake it sits in. They really wanted to dig that hole and yet most of the its contents are sitting in those big blue piles.

Finally the oldest example in the world, the Big Hole in Kimberley. The piles of dirt were put back in the hole, or covered in houses. Mining underneath here reached a kilometre depth.

We’ve been looking at some famous diamond mines. Diamonds form deep in the earth and those worth digging holes for only reach the surface via weird fizzy molten rock called kimberlite. This magma zips up from 100km depth to the surface in only hours. Travelling at depth along a narrow crack (or dyke in geo-speak1) when the magma reaches the surface it forms a carrot-shaped pipe. The magma solidifies, peppered with diamonds that formed at depth and were pulled up inside it.

The pipe is circular in cross-section, so as the miners dig out the kimberlite they leave a circular hole. The vast majority of what is dug out is waste – only the precious diamonds are extracted. Kimberlite is bluish in colour, as you can see from the piles of it above.

The Big Hole in Kimberley was the first kimberlite pipe ever identified2, in the 19th Century. Diamonds found before then were from placer deposits, river gravels that contain diamonds eroded out of kimberlite pipes. Diamond placer deposits were first discovered in India and then Brazil. But the biggest area for modern mining is on Namibia’s Skeleton Coast.

Southern Africa’s Orange river rises on the Kapvaal Craton, an area rich in kimberlite pipes. For 100 million years it has flowed across the continent into the Atlantic ocean, leaving thick placer deposits. These have since been pushed around since by wind and ocean waves to cover a wide area.

Starting at the beginning of the Twentieth Century, German settlers found diamonds and the government designated a huge area of land as Sperrgebiet – forbidden territory. This whole area remains closed, but active mining is concentrated the southern end, on the coast, where the diamonds are concentrated in ancient beach sands.

The mine here looks very different from the others, no round hole or blue kimberlite (but look for the regular patterns on the spoil heaps). Like the other mines, this one was caused by the desire for the beauty and strength of diamonds.

]]>http://all-geo.org/metageologist/2015/02/looking-from-the-sky-at-diamonds/feed/0Great Geology in Google Maps: mapping from abovehttp://all-geo.org/metageologist/2015/02/great-geology-in-google-maps-mapping-from-above/
http://all-geo.org/metageologist/2015/02/great-geology-in-google-maps-mapping-from-above/#commentsSun, 08 Feb 2015 11:00:15 +0000http://all-geo.org/metageologist/?p=3899Continue reading →]]>In most cases, geological maps are made by piecing together observations of hundreds of individual outcrops. Boundaries between types of rock are covered in grass and sheep1 and have to be traced on the map later as a line between rock outcrops, like a inverted game of dot-to-dot. In areas like Himalayas the same boundaries may be visible in an instant on a vast wall of rock. Quickly mapping vast areas of country by tracing features by eye across expanses of bare rock is a great way to do geological mapping.

It turns out you can have a go at this ‘Himalayan-style’ mapping at home. Make a cup of tea (add salted butter, or spiced sweet milk for authenticity) and fire up Google Maps. It works outside the Himalayas too – deserts are great for this.

This is an area of Namibia in SW Africa. Immediately you can see different areas of rock – the image itself is like a geological map, only without labels. Himalayan geologists would identify the different areas of rock by getting samples, but we’ll stick with Googling the geology of the area.

Let me take you through the geological components and their history.

First off, the orange lines that look like rivers. Ignore them: they are rivers and we are geologists, not geographers. Mentally filter them out of the image.

Next look at the stripy area top left – we’ll call them the Damara sediments. They formed in the long-vanished Khomas ocean between the Congo craton (ancient piece of crust) and the Kalahari craton. The area is stripy because the sedimentary layers are not flat. They are now on edge and form ridges on the surface.The light orange area far right is made of Damara granites (and modern sand, again filter it out, if you can). These formed when the Khomas ocean closed and Congo and Kalahari got stuck together as part of Gondwanaland. This happened about half a billion years ago.

Bottom left, the rusty brown area is sediments and volcanic rocks that are part of the Karoo Supergroup. These formed on Gondwanaland between the Carboniferous and the Jurassic when geological conditions in this part of Africa were relatively calm.

The really obvious round thing that I’ve perversely left until last is a circular granite intrusion called Brandberg that formed a little later, as Gondwanaland was ripped apart and the South Atlantic opened.

I’ve given you the relative ages of the different rock packages, but we could have worked this out from Google Maps. The observations I’ll show you are exactly the same as those used by field geologists. What follows are a sequence of close-ups. Some are outside of the original area, but none are far away.

Unconformity

We’ve got two areas of sediments, the Damara and the Karoo. How do we know one is older than the other?

Here we see the red-brown Karoo sitting in a blob in the middle, with stripy purple-brown Damara to the NE. Look carefully (zooooming helps) and you can see circular lines in the Karoo. Note that the rivers go round this blob.

We are looking at a hill of Karoo sediments. The lines are the edges of different beds and they are tracing contours around the hill. These are flat layers of underformed sandstone2.

Try tracing the lines in the Damara – these are also the edges of sediment beds but they have been tilted. Note that they are cut by the edge of the Karoo. The surface between the two rock packages is an unconformity. It represents the time when the Damara sediments were pushed into a mountain belt and then eroded into a flat surface.

Folding and cross-cutting granites

This is a view of the contact between the Damara granites and the Damara sediments, which have some zig-zaggedy folding here. The boundary between the two is fairly abrupt and cuts the folded bedding planes. Using the principle of cross-cutting relationships the granite is younger than the sediments and the folding (although the folding may be related to the emplacment of the granite). There are some fine white lines crossing the folded sediments that may be veins of magma that came out of the main granite – further evidence of what came first.

Scoot around a bit and you can see that there is an unconformity between this older Damara granite and the Karoo, which gives us the relative ages of the three.

Three-way contact

This is a view of three of our rock packages. There’s a prominent river forming a yellow bar across the picture. Apart from that, from left to right we have: Damara sediments; Karoo sediments, sitting above the unconformity; the Brandberg granite. The boundary between the Karoo and the Brandberg is straight and cuts across bedding, suggesting the intrusion is later.

Another possibility to consider is that this is an older granite and the edge is just the edge of a hill. Well, look a the patterns of the streams – they are flowing down from the centre of the granite (it actually forms the biggest mountain in Namibia). For it to be higher now than the flat Karoo sediments, it must be younger than them.

The boundary between the Brandberg and the Karoo is quite interesting3, with tilting of the Karoo and a zone where the sediments and granite are mixed. Apart from the small zone that is Karoo coloured but without clear bedding, I can’t make this out.

The geological history I started with is based on many things, including precise dating of events and detailed field work. However, the basic age-relationships between the rocks can be worked out using simple geological rules and good photographs.

These same principles are being used on other planets where geologists have never been. We know a lot about the geological history of Mars by mapping from space using just these techniques.

Damara granite next to folded Damara

]]>http://all-geo.org/metageologist/2015/02/great-geology-in-google-maps-mapping-from-above/feed/1Hot spot volcanoes: no plumes required?http://all-geo.org/metageologist/2015/02/hot-spot-volcanoes-without-mantle-plumes/
http://all-geo.org/metageologist/2015/02/hot-spot-volcanoes-without-mantle-plumes/#commentsSun, 01 Feb 2015 11:46:30 +0000http://all-geo.org/metageologist/?p=3889Continue reading →]]>It’s a simple and well-known picture. Volcanoes form either at plate boundaries due to subduction or inside plates due to mantle plumes. Invoking plumes – columns of hot rock rising from deep in the mantle – is an awfully useful way of explaining oddly-placed volcanoes, both ancient and modern.

Too useful, many people think. The concept has been abused. See Erik Lundin’s excellent critique in “52 things you should know about Geology“: “A concept that is granted the freedom of perpetual ad hoc amendments has the ability to explain anything … But such a concept can neither be falsified not used predictively. In the long run it may be wiser to ask yourself ‘Is there an alternative explanation?’ rather than simply shrugging, ‘Plumes do that’.

Layers of volcanic debris and ash from the Newer Volcanics Province, Australia. Spot the bombs. With permission from Stephanie Sykora

How else to melt the mantle?

The best place to find alternative explanations is mantleplumes.org a site dedicated to “discussing the origin of “hotspot” volcanism”. The site lists many mechanisms, but I’m going to focus on just two: edge-driven convection and shear-driven upwelling.

It’s not that hard to melt the mantle. It’s everywhere fairly close to its melting point and it gets hotter the deeper you go. A key point to understand is that most of it only stays solid because of the intense pressure it is under. As mantle quickly rises up beneath mid-ocean ridges it melts because it stays hot as the pressure reduces. All the atoms that were squeezed tightly together in solid crystal lattices manage to break free into a liquid state, once the earth’s grip lessens a little.

Almost all matter behaves like this, but it doesn’t feel like common sense because we are most familiar with the freezing and melting of water, which is weird and works the opposite way round (which is why ice floats). I labour the point because both of today’s mechanisms are ways of creating upwellings1 – areas where hot mantle material rises up and so is prone to melting.

Edge-driven convection (EDC) is flow caused around the edges of continents. Continents have deep cold roots to them, like icebergs2. A convection cell is set up with a zone of upwelling about 600km from the craton edge. It wouldn’t be surprising if you find some volcanic rocks above here.

The above model assumes nothing is moving, but we know that there will often be flow of the mantle relative to the plates. If there is mantle flow across an edge (for example a craton edge) then material will flow up. This is one way of producing shear-driven upwellings (SDU)3

Diagram showing mechanisms of shear-driven upwelling. I discuss the left-hand example. Taken with permission from mantleplumes.org.

So far, so theoretical. Let’s go to Australia and look at some rocks.

Welling-up down-under

The Newer Volcanics Province is an active (but dormant) volcanic area in Victoria, Australia. To get a great overview of its many great volcanic features, check out this post (from which the photos here come). The lava is basaltic in composition – just what you’d expect from melting of mantle, but we are a long way from a plate boundary.

A recent paper in Geology studies what’s going on deep beneath the lava. Rhodri Davies and Nicholas Rawlinson of ANU, Canberra and Aberdeen universities start off with a spot of 3-D seismic tomography. Previous workers through they could dimly see a plume beneath, but armed with a new seismic data set from the (wonderfully-named) WOMBAT project they show there is no plume. Instead they show a shallow low-velocity anomaly underneath the NVP, consistent with region of hotter mantle, perhaps containing a small proportion of magma.

A piece of mantle that flowed upwards and melted: “Volcanic bomb with a olivine-rich xenoliths from the mantle at Mt. Noorat – Victoria, Australia” Courtesy of Stephanie Sykora

Having made the plume vanish, they turn to modelling of the mantle flow, based on their new improved knowledge of what is down there.

This area of Australia sits outside of the deep cratonic root. It’s like a thin ledge sticking out from the side of the iceberg. Therefore the edge of the deep root, that might cause EDC is to the north, allowing the upward return flow to sit directly beneath the NVP. Their models also include relative plate motion (how the plate is moving relative to the mantle beneath). This allows them to model the effects of SDU as well.

The modelling results produce a region of upwelling with velocities between 1 and 2 cm a year – fast moving for mantle – sitting directly underneath the NVP. This neatly explains the NVP, without any need to invoke plumes.

The mechanism is neat, but begs the question as to why there isn’t a line of volcanoes all around cratonic roots. Addressing this question, they point out the interaction of SDU and EDC. Under the NVP the two effects are complimentary – upwelling is increased where the mantle is flowing away from step. Also the edge here isn’t straight – 3D effects are important. Finally, mantle composition varies. So-called ‘fertile’ mantle may melt under conditions where mantle that’s already had melt extracted would not.

Are plumes dead?

There’s a compelling model here for explaining many volcanic hot-spots around the world with no need for plumes. Do we need plumes at all? Gillian Foulger, the force behind mantleplumes.org certainly doesn’t think so. Also Don Anderson of Caltech who recently had the posthumous last word at the AGU annual meeting last year.

Their views may prevail in time, but for the moment most of us still believe in plumes. Explaining how small-scale convection causes a minor volcanic field in one place doesn’t explain continental flood basalts like the Deccan or Siberian Traps. You know, the ones that cause mass extinctions and thickly cover vast areas.

But clearly plumes and not the only game in town. To progress, ideas involving plumes need to be anchored in our understanding of the deep earth, to be falsifiable and have predictive power. Recent research aims to do just that. Watch this space.

References

]]>http://all-geo.org/metageologist/2015/02/hot-spot-volcanoes-without-mantle-plumes/feed/1Great Geology in Google Maps: duneshttp://all-geo.org/metageologist/2015/01/great-geology-in-google-maps-dunes/
http://all-geo.org/metageologist/2015/01/great-geology-in-google-maps-dunes/#commentsTue, 27 Jan 2015 18:21:45 +0000http://all-geo.org/metageologist/?p=3895Continue reading →]]>Google Maps is a great resource, particularly in satellite view. My favourite way to enjoy it is via the Chrome extension “Earth View from Google Maps“. This pops up a gorgeous image in every new tab. Many show human landscapes, but every now and then one appears that catches this geologist’s eye. This post is the first in a series exploring and celebrating these images.

This view is of the Rub’ al Khali or ‘Empty Quarter’ of the Arabian Peninsula – the largest sand desert in the world.

In dry environments sand is moved not by water but by wind. The characteristic landform is the sand dune. Common in deserts on earth, they are also found on Mars and even comets.
The shapes of dunes depends on the supply of sand, but above all the wind. Wind strength and direction, averaged over the year, will determine the shape of a dune. Various types of dune are recognised. The ones in this image are complex. Further east of here the dunes are clearly linear features, but here the lines are broken up into loops reminiscent of arabic script.

Look carefully and you’ll see that there are dunes upon dunes. The surface of the large sand bodies are covered in ridges and patterns with a wide variety of shapes. These themselves may have small ripples on, only visible if you visit in person.

This image is from the ‘Grand Erg Oriental’ in the Sahara desert in Algeria. It shows star dunes, which form when the wind is variable and simply piles the sand up into mounds 100s metres tall.

Both these areas of sand (by chance) sit above oil-fields, the oil-bearing rocks sitting deep under the ground. Ancient desert sands are of interest to oil geologists as the grains are very round and form sandstones that can contain a lot of liquid.
Ancient ‘aeolian sandstones’ are basically fossilised sand dunes and are often red. The ‘red sandstones’ of the UK, much Triassic sandstone in Europe and the classic Navajo Sandstone in the US are of this type.

Next time you see a red sandstone with big swooping cross bedding, think of these pictures.

Like experts blind-tasting a glass of wine and recognising where it came from, geochemists studying the deep earth aim to find out where a particular liquid came from. Their liquid – basaltic magma formed from melting of the mantle rocks – is now solid, so ‘tasting it’ involves dissolving it in Hydrofluoric acid or vapourising it in the bowels of a machine with an unlovely name.

A wine buff can sniff out where a wine came from because they’ve already sampled lots of known vintages. Geochemists have a much harder job – basalt samples don’t have labels. They are formed from melting of the rocks below, but was the material that melted from the deep earth or shallow? Is it from oceanic crust that’s been subducted and remelted or material that’s sat around since the earth was formed?

Mantle geochemists still have more questions than answers, but that’s because what they do is really hard. They are like first-time wine-tasters who’ve been given anonymous bottles and only a fuzzy satellite image of France to work with.

What to sample?

Most basalt is produced at mid-ocean ridges, where oceanic plates move apart and the underlying shallow mantle rises up, decompresses and melts. Known as MORB1 this is plonk. Widely produced, homogenised and of little interest to the true connoisseur.

Basalt from oceanic islands (OIB) is mantle geochemists’ favourite tipple. Found only in select areas far from plate boundaries it has many flavours but can be distinguished from MORB by a trained nose. Thought to be formed by material rising up in hot plumes from the deep mantle it carries whiffs of what is lurking down there.

Of particular interest at the moment are dark intense picritic lavas. Formed under higher temperatures in smaller batches they tell us more about what happens when a mantle plume first nears the surface.

Tasting

The process of producing basalt from the mantle is complex, depending on the composition and mineralogy of the melting material plus the depth and pressure. Also a lot may happen to the magma before it cools as the surface as lava. Iceland has rhyolite lava flows – very different in composition to basalt, but ultimately formed from mantle melt2.

So to study the mantle that was melted tasting the basic chemistry of the lava is not enough as changes due to later processing can obscure the smell of the source material. More sensitive mechanical noses are required, that can sniff trace elements or isotopes that may be unchanged by later processing and hold the tang of the source mantle.

Terroir

Mantle composition, as inferred from basaltic melt, is very variable, leading to the identification of a ‘zoo’ of acronyms, from DMM and HIMU (sources for MORB) to EM and FOZO (for OIB).

A key concept is ‘enrichment’. Particular elements are ‘incompatible’ which means that if they are in a rock that melts, they are strongly partitioned into the melt. As the ‘enriched’ melt moves away you are left with a ‘depleted’ residue. Continental crust is extremely enriched, oceanic crust less so.

For this reason the churned up mantle contains portions which are depleted by having had oceanic crust melted from it (DMM) and other enriched portions which contain recycled oceanic crust (HIMU). Small amounts of continental crust may enter the mantle – perhaps the mantle frozen to the base of continents may fall off. Also continental material (sediment, stones frozen into icebergs, the Titanic) may end up on ocean floor destined to be subducted. EM and FOZO are sources that may have been enriched in this way.

Primitivo

Geochemists don’t just worry about the mantle, but the whole earth. Chondritic meteorites have long been thought to be a model of the bulk chemistry of the earth. Strip out iron and other elements into the core, account for the enriched crust and you can calculate the bulk composition of the mantle.3.

Compare known mantle compositions with the theoretical bulk composition and you get a gap, leading to the idea of a hidden reservoir of ‘primitive’ composition (e.g. closer to chondritic). Conceptually this is similar to the idea of ‘dark matter’ in physics – a thing invented to explain inconsistent pieces of evidence, but for which there is no direct evidence. Only time will tell if hidden reservoirs in the mantle will be found or go the way of the luminiferous aether.

Basalt in a vineyard

Paradoxes and problems

The idea of hidden reservoirs was extremely popular over 20 years ago, when it seemed that subducting plates stopped at 660km depth, where a ‘phase change’ in minerals alters the stiffness of the flowing mantle. This suggested that the lower mantle could be of very different composition. But modern seismic imaging suggests whole-mantle convection is possible, suggesting that over billions of years the mantle will have been thoroughly stirred – with the exception of a mysterious layer at the base of the mantle.

Mantle geochemists often talk of ‘paradoxes’ – patterns of ratios between elements and isotopes that aren’t consistent. There is a lead paradox, and an Argon one, plus a ‘heat-Helium imbalance’. Explaining these in terms of a primitive reservoir is one way, but others are possible. Let’s look at Helium.

Helium comes in two flavours. The first 3He is just two protons and a neutron and from the earth’s point of view it’s primoridal, it’s always been there and never changes. In contrast, when the great hulking nuclei of Thorium and Uranium fall apart they leave small fragments – making 4He in the alchemical process of radioactive decay.

The ratio of the two Helium isotopes is fairly consistent for MORB sources, but wildly variable for OIB. Material with a high ratio has been interpreted in terms of a primitive reservoir, rich in primoridal 3He. An alternative explanation is that the source is extremely low in 4He due to it being depleted in Uranium/Thorium. Or maybe the 3He bubbled up from the core.

Tasting the earth does not give you all the answers, but it is vital part of the picture. As I continue my tour of the deep earth, geochemistry will often have an important role to play. The difference between OIB and MORB is a powerful argument in the armoury of those who favour mantle plumes and as seismologists start to see odd things at the base of the mantle, getting a whiff of the chemistry here becomes very important.

Tasting ‘black cherries’, ‘tar’ or ‘cat-pee’ in wine is a clever trick. Tasting blobs of 4.5 billion year-old rock or recycled oceanic crust in basalt is even cleverer. Cheers!

Further reading

]]>http://all-geo.org/metageologist/2015/01/tasting-the-earth-mantle-geochemistry/feed/0Metamorphic petrology under stress: round 2http://all-geo.org/metageologist/2015/01/metamorphic-petrology-under-stress-round-2/
http://all-geo.org/metageologist/2015/01/metamorphic-petrology-under-stress-round-2/#commentsFri, 02 Jan 2015 17:16:25 +0000http://all-geo.org/metageologist/?p=3860Continue reading →]]>Back in August I wrote about an extremely important paper by John Wheeler of Liverpool University called “Dramatic effects of stress on metamorphic reactions”. This uses a theoretical approach to show that differential stress (squashing rocks) is a very important control on metamorphic reactions. If true, this would imply that many estimates of depth of metamorphic conditions (that ignore the squashing) are wrong. Maybe eclogites don’t form at great depth after all.

Counterblast

Conceived as a provocative paper, it’s no surprise to find a “Comment” on it in the latest edition of Geology. Written by Raymond Fletcher of Penn State, it aims to “show
that Wheeler’s claims do not have a sound basis” by constructing a more complete mathematical model “for metamorphic reaction and pressure solution” (the two processes that Wheeler’s original paper wound together).

For both our sakes, I’m not going to get into the detail of the mathematics (all papers are open-source, so you can read it yourselves). It isn’t massively complicated maths – single lines of algebra only – but what matters here is the assumptions and simplifications made and whether they are valid.

Fletcher’s comment picks on one aspect of the original paper – that in the section quantified the effects of differential stress, it focussed on a single way in which atoms can rearrange themselves called incongruent pressure solution. Fletcher’s set of equations are a more complete model that shows that Wheeler’s results are merely a ‘special case’ leading to ‘contrived outcomes’.

A spirited defence

Wheeler is uniformly polite and positive. He starts by thanking Fletcher for his stimulating Comment and listing the ways in which they agree. Then this:

“But it is inappropriate to say that I am wrong, first because his model is not of the incongruent pressure solution (IPS) pathway, second because it actually containsconfirmation of some of my claims, and thirdly because it is extremelyrestricted in scope.”

He then proceeds to show that Fletcher’s model doesn’t just model a single pathway and the he lists the assumptions made by Fletcher and demolishes each one. For this audience member, Wheeler starts to win the battle by the depth of context he brings to the discussion.

For each assumption he refers to existing research into real world complications. These include: defining 3-D stress as a single term in an equation is complicated – taking the simple average of the 3 dimensions is not correct; fluid pressure may control reactions, not stress; the topology of grains is important and the one chosen by Fletcher extremely unrealistic; diffusion of atoms is often a limiting factor in metamorphism; porphyroblasts often grown in specific shapes – ‘interfacial’ kinetics may also be important.

For this (slightly biased) reader the knock-out blow was the fact that 3 times the research into these complications is his own allowing Wheeler to write that Fletcher “may well have rediscovered the sorts of problems described above (Ford and Wheeler, 2004) but by ignoring these he reduces the value of his assertion that…” .

All good replies to comments look to the future:

“In summary, Fletcher’s model is too restricted in scope to undermine my conclusions: we agree that a more general model is required. I challenge him and other interested readers (including myself) to construct such a model, which would be of great benefit to understanding how metamorphism and deformation interact.”

Science in action and in the open

I remind you again of the important implications of Wheeler’s paper – existing estimates of metamorphic conditions – used to build tectonic models – are suspect. To quote another article in Geology discussing it “the potential inaccuracy of depth estimates based on minerals would question current paradigms in geology“.

Wheeler’s original paper was a huge challenge to metamorphic petrology. It has withstood the first attempt to refute it. This is science in action, in open source papers for all to view. I hope you’ll read the papers yourself and we can follow the unfolding story together.

]]>http://all-geo.org/metageologist/2015/01/metamorphic-petrology-under-stress-round-2/feed/1Seismology in spacehttp://all-geo.org/metageologist/2014/12/mars-moon-sun-seismology/
http://all-geo.org/metageologist/2014/12/mars-moon-sun-seismology/#commentsTue, 09 Dec 2014 21:18:55 +0000http://all-geo.org/metageologist/?p=3829Continue reading →]]>Seismology – using the propagation of waves through bodies to work out their internal structure – is extremely useful. You can use it to find oil, track active faults or understand what is at the centre of the earth. The principles and mathematics developed by studying the earth apply to other bodies too. The Moon, Mars, even distant stars: seismology can help us understand these bodies also.

Moonquakes

Let’s start close to home. As part of the Apollo moon-landings a series of seismometers were installed and collected seismic data for nearly 8 years. The vibrations were caused by small moon-quakes and meteorite impacts. To help things along (and to assist with calibration) a few pieces of rocket and the ascent stages of several lunar modules were deliberately crashed into the moon (once they were no longer needed, as the NASA page helpfully points out).

The moon is not tectonically active in the way the earth is – most moonquakes had Richter scale magnitudes of less than 2. Events not caused by collisions were clustered on a monthly cycle, suggesting they were caused by changes in tidal forces as the moon orbits the earth. The discovery of some recent tectonic features (found in imaging from the Lunar Reconnaissance Orbiter) suggests something else is going on. Perhaps cooling and contraction of underground melt is causing these surface features to form.

This precious data was recently re-processed using the latest seismic techniques to tease out new details of the lunar interior. The seismic data was ‘messy’, due to smearing of signal in the upper 20 km of the crust, which is heavily fractured due to meteorite impacts. Modern processing allowed a clearer picture of the moons interior to be taken. It contains a metallic core, partly molten, but also a layer of molten rock at the base of the mantle.

Knowing the interior of the Moon is important for understanding the earth too. The most popular model for the Moon’s origin involves a massive impact between the earth and another body. We need to know what ended up in both the Moon and the earth to understand this process. It’s also interesting to reflect on the fact that the Moon is smaller than – and so would cool faster than – the earth. So why is it molten at the base of the mantle and the earth is not? A recent paper suggests the molten layer persists due to frictional heating of the moon from tidal forces. The same process (but stronger) heats up the moons of Jupiter, for example creating volcanoes on Io1.

Marsquakes

Mars is the only planet inhabited solely by robots2. The first Martian robot3, the Viking lander, had a seismometer stuck on the leg. Sadly it wasn’t very sensitive and didn’t detect any marsquakes at all (just a lot of wind). The NASA InSight mission aims to put this right, landing sensitive instruments (including a seismometer) in 2016.

Starquakes

There are no seismometers on the sun – they wouldn’t survive long4 – but it turns out that the same principles and mathematics we use to probe the earth also work on stars.

Asteroseismology is a form of seismology that uses pulsations in the light from stars to infer their internal structure. The outermost portion of a star is extremely turbulent and causes the entire star to vibrate like a dog waiting for a stick to be thrown. Instead of measuring the vibrations directly, we infer them from tiny variations in the intensity of the light, which we can measure from the delicious cool of our planet.

Sound waves (P-waves to seismologists) can be measured in both planets and stars. The sun has more exotic types of waves too and using these together gives us an invaluable view of the star’s internal structure. This in turn can be used to infer its age.

Similar techniques have been applied to Jupiter, which is also a ball of gas with something mysterious inside. Here the waves are detected by direct viewing of the surface5, as illuminated by the sun. The picture we’ve got so far is still rather fuzzy, but consistent with what we’d already guessed was there (a rocky core surrounded by metallic Hydrogen and then a Hydrogen and Helium atmosphere).

The Kepler missions search for planets orbiting other stars is linked to asteroseismology. The planets are discovered by the faint dimming effect as they pass in front of stars (the transit method). To do this we need to accurately know the size of the star and asteroseismology is the best way to do this. The ability to know the age of the star is useful too. Only a planet circling a relatively old star will have had enough time for life to evolve. The dream is of course to find a planet with intelligent life. If there is one, they are surely doing seismology. It’s such a useful technique to understand what lies beneath the surface of things.

]]>http://all-geo.org/metageologist/2014/12/mars-moon-sun-seismology/feed/1The Constitution of the Interior of Earth, as Revealed by Earthquakeshttp://all-geo.org/metageologist/2014/12/seismic-tomography-earthquakes/
http://all-geo.org/metageologist/2014/12/seismic-tomography-earthquakes/#commentsMon, 01 Dec 2014 12:06:16 +0000http://all-geo.org/metageologist/?p=3820Continue reading →]]>How to tell if the loaf of bread in your oven is cooked? You can see the outside is nicely browned, but you can’t see the middle – is it doughy still? Give it a tap and listen. If it sounds hollow then it’s ready. The sound of the tap passes through cooked bread differently than through dough.

As with loaves of bread, so with the earth beneath us. Earthquakes give the earth a tap and we listen to the earth with seismometers, but the principles are the same. The sound and shaking caused by the tap passes through places we can’t ourselves get to and the way they are changed tells us about the material they’ve passed through.

Simple seismology

Every time an earthquake ‘taps’ the earth, it sends seismic waves whizzing off in all directions. For those of us who live on the outside, the most dangerous types are those that travel along the surface, but we are interested here in body waves, the ones that travel through the earth’s interior. Imagine yourself watching a seismograph when an earthquake’s effects arrive1. The first set of twitches2 are caused by the arrival of the primary or P-waves. These travel fastest and work like sound waves – by compressing material in the direction of travel. Next arrive the secondary or S-waves which move forward while shearing material side to side (think of a slithering ssssnake).

What earthquakes can reveal

The first person to clearly identify P, S and surface waves on a seismogram3 was British scientist Richard Dixon Oldham. Born in 1858, he was child of the Empire. His father was Professor of Geology at Trinity College, Dublin. Following his father’s footsteps led him to the Geological Survey of India. Studying a 1897 magnitude 8.1 earthquake in Assam he spotted the distinct arrival of P, S and surface waves, due to their travelling at different speeds. What’s more, he linked these observations both to existing theory of how waves propagate in materials and to fault ruptures on the earth’s surface.

Ill-health took him back to Britain in 1903, where his research continued. In a lovely 1906 paper called “The Constitution of the Interior of Earth, as Revealed by Earthquakes” he showed what seismograms could tell you about the very centre of the earth.

In the introduction, he paints a picture of woe:

“Many theories of the earth have been propounded at different times: the central substance of the earth has been supposed to be fiery, fluid, solid, and gaseous in turn, till geologists have turned in despair from the subject, and become inclined to confine their attention to the outermost crust of the earth, leaving its centre as a playground for mathematicians.”

Taking data from a mere 14 earthquakes and similar number of stations, he plotted the interval between earthquake and wave arrival4 against distance in ‘degrees of arc’ (180 degrees of arc would be a wave that had passed directly through the earth to be measured at the point opposite the earthquake).

Figure 1 from Oldham (1906). The lower line is P-waves, the upper S-waves

Oldham’s insight, rescuing the centre of the earth from the mathematicians, was to use the obvious change in the pattern at about 150 degrees to infer a core of very different composition from the surrounding material. A seismometer at greater than 150 degrees from the earthquake must trace rays that have passed through the deep earth – so the times it records tell us about the very core of the earth.

Figure 2 from the same paper. Note how the paths taken by seismic waves are refracted by the changes in velocity at the core-mantle boundary

Oldham was confident that his data showed that “the central four-tenths of the [earth’s] radius are occupied by matter possessing radically different physical properties” and time has proved him right.

Seismology across the globe

Reading this paper is reminder that globalisation is not a modern phenomena. Oldham was drawing on recording stations from many place – not just from British Empire (New Zealand, Australia and South Africa) but also the Americas and the Russian Empire (Irkutsk, Tashkent and Tiflis). He refers to literature from Japan and papers written in both German and Dutch. The latter involved field work from the Dutch East Indies (now Indonesia).

While scientists across the world were communicating with each other, this was not necessarily done quickly. All the earthquakes he studied were between 12 and 4 years old.

A shadowy view of the inner core

As speculated upon by Oldham, but proved by others in the next few decades, S-waves don’t travel through the core, because it is liquid – seismometers on the opposite side of the earth don’t see an earthquakes S-waves as they are in the ‘shadow’ of the earth’s core. The patterns of refraction you see in Oldham’s diagram above mean that there is a shadow-zone for P-waves also.

In 1936, taking data from within the shadow zone of an earthquake in New Zealand, Inge Lehmann found waves that shouldn’t be there. Tracing their paths, she explained them by inferring a distinctive solid inner core that created more ways for rays to reflect and refract their way through (for more detail see this great write-up).

Seismic tomography

Seismic data can used for many things. Locating earthquake foci helps us trace patterns of faults across the globe and so better predict which areas of the surface are at risk. Making bangs at the surface and measuring the reflections from shallow sub-surface layers is a great way to find oil and gas. Using them to better understand the structure of the deep earth is known as global seismology, or seismic tomography.

The key principles of using seismology to study the earth’s interior were set in the early Twentieth Century. The time taken for waves to pass through the earth gives the speed they travelled, which tells you something about the properties of the material they passed through. Building up models of the interior of the earth allows you to trace the paths they took. Studying sets of paths that go through the same portion of the earth tells you something about the properties of that part of the earth.

Image of the deepest mantle from seismic tomography. Part of figure 7 from Steinberger et. al 2012

Armed with millions of data points and highly sophisticated mathematical models modern seismologists are able to image the earth’s depths in detail, down to scales of a hundred kilometres. As well as understanding how things change with depth, they are also able to spot differences between different places at the same depth. The very base of the mantle in particular contains a lot of distinct areas with unusually low velocity.

Variations in seismic velocity at the same depth can be explained by variations in temperature or in composition. Either way, to explain the features seismology shows us, we need to bring some other sciences to bear. Let’s move to chemistry next.

]]>http://all-geo.org/metageologist/2014/12/seismic-tomography-earthquakes/feed/4Subduction is not the endhttp://all-geo.org/metageologist/2014/11/subduction-is-not-the-end/
http://all-geo.org/metageologist/2014/11/subduction-is-not-the-end/#commentsSun, 09 Nov 2014 10:29:31 +0000http://all-geo.org/metageologist/?p=3680Continue reading →]]>Subduction is just the beginning. Stuck on the surface of the earth as we are, it’s easy to think that when oceanic lithosphere1 is destroyed when it vanishes into the mantle. But this is wrong. The more we manage to peer into the earth’s depths, the more clearly we see that subducted oceanic lithosphere is still down there. It’s arrival into the deep earth influences many things and sets of changes that in turn affect the surface.

Our growing understanding of what happens to oceanic lithosphere when it reaches the deep earth comes from the integration of a vast repertoire of scientific approaches. Physics, chemistry, mathematical modelling, high-pressure laboratory experiments, tiny grains in diamonds – all these things have something to tell us about the deep earth.

As in the deep oceans, so in the deep earth. We now know that both places are extremely varied environments, warped by extreme pressures, full of the weird and the exotic and some surprising relics from the past. Both are hard (or impossible) to visit, yet they still have influence over surface conditions.

A journey into the deep

This is the first of a series of posts on the deep earth. There’s a great mass of fascinating recent science out there that deserves to be better know. As a taster, let me tell you about lovely little Nature paper from 2005 that picks out the themes I hope to trace through future posts: how much of the deep earth is still mysterious, how we know what we know, the billion-year timescales and how the surface and depths are intimately linked.

In the paper David Dobson and John Brodholt (both, then as now, at University College, London) speculate that some distinctive parts of the deepest mantle are formed from subducted banded-iron formation.

Folded banded iron formation. Photo was not taken at the core-mantle boundary. Image from Wikipedia.

Seismic studies (tracing the shakings caused by earthquakes) have identified parts of the lowest mantle where earthquake waves pass through over 10% slower than expected. This means that these areas, known here as ultralow-velocity zones (ULVZs), must be in some way different – either they are made of different material or they are are at a different temperature to their surroundings.

Nobody knows what the ULVZs are made of, but our authors put the case that they contain large volumes of banded-iron-formation (BIF). Between 2.8 and 1.8 billion years ago photosynthetic organisms (probably bacteria) profoundly changed our planet’s atmosphere by producing vast quantities of Oxygen. One consequence was that ferric iron, dissolved in the ocean, bound with the Oxygen ‘waste’ and was precipitated out as solid minerals of magnetite and haematite. This process created vast volumes of marine sediments rich in iron. These banded iron formation deposits are found in rocks of this age from around the world.

Assuming this material was subducted what would happen and could it account for the properties of the ULVZs at the bottom of the mantle? Tracing through the required steps, our authors show that BIF material would be dense enough to sink to the base of the mantle, able to separate out from the more normal material in the subducted plate and would not dissolve into the underlying iron-rich core. If these steps had occurred, then the material would match the measurable properties of the ULVZs.

Just because something is plausible, doesn’t make it true of course. Proving this link between ULVZs and BIFs definitively may never be possible. But one further clue exists: having an iron rich layer close to the edge of the core might help explain patterns of change in the length of the day we surface-dweller can observe.

The earth’s nutationsare daily variations in its rotation. They are caused by the pull of the earth, sun and other bodies, but are also influenced by the degree to which the core and mantle spin independently or are coupled together. The outer core is liquid and the mantle solid, so they would be able to spin independently, but studies of nutations suggest there is some coupling – transfer of momentum – between them. If ULVZs are formed of BIFs and so very rich in iron, they would interact with the earth’s magnetic field causing electromagnetic coupling in a way that matches the observations.

It’s a mind-expanding idea. Sediments formed billions of years ago in a now alien atmosphere sit thousands of kilometres under our feet, subtly modulating the length of the day.

Deep cycles

When listening to a classical orchestra, most often we are paying attention to the high to middling frequency sounds – the violins, flutes and so on. The full experience – a live performance – is vastly enriched by the deepest, lowest frequency instruments. A full desk of double basses, or some bass clarinets make the air itself throb, producing a more physical, visceral and emotional experience2.

Our experience of living on this planet is enriched by our understanding of its cycles of change. The most frequent are most apparent: the year’s seasons; the carbon cycle; Milankovitch cycles that bring ice to sculpt the landscape. Subduction is one of the deepest, the slowest of them all – linking the familiar surface with the mysterious depths, it is the fundamental frequency in the orchestra of the earth.